Isolated and impoverished North Korea says it is ready for its proposed long-range rocket launch, with preparations being made overnight to inject fuel into its tanks.

The announcement sparked immediate condemnation from South Korea and Russia and a plea from China, its main ally, for calm.

The United States and its allies say the launch of the Unha-3 rocket is a long-range ballistic missile test, but North Korea insists it will merely put an Earth weather satellite into space.

Regardless, the launch breaches UN sanctions imposed to prevent Pyongyang from developing a missile that could carry a nuclear warhead.

Russia, a former backer of North Korea which has boosted economic ties with Pyongyang, denounced the program.

"We consider Pyongyang's decision to conduct a launch of a satellite an example of disregard for UN Security Council decisions," Russian foreign ministry spokesman Alexander Lukashevich was quoted as saying.

North Korea defended the launch as a sovereign right.

"The weight of our satellite is 100kg. If it was a weapon, a 100kg payload wouldn't have much of an effect. Our launching tower is built on an open site," said Ryu Kum-chol, vice director of the space development department of the Korean Central Space Committee.

Mr Ryu said that the rocket assembly would be complete on Tuesday.

The launch is set to take place between Thursday and next Monday around the 100th birthday celebrations of the founder of North Korea, Kim Il-sung, whose grandson, Kim Jong-un, now rules. Kim Il-sung died in 1994.

"The launch of the Kwangmyongsong-3 satellite is the gift from our people to our great leader, comrade Kim Il-sung, on the occasion of his 100th birthday, so this cannot be a missile test," Mr Ryu added.

The West says the launch is a disguised ballistic missile test by a country which walked out of so-called six-party disarmament talks three years ago.

South suspects nuclear plan

South Korea, which remains technically at war with the North after their 1950-53 conflict ended with a truce rather than a treaty, warned Pyongyang it would deepen its isolation if it went ahead with the launch.

Security sources in Seoul, citing satellite images, have said that North Korea is also preparing a third nuclear test following the rocket launch, something it did in 2009, a move bound to trigger further condemnation from the West.

"It is disappointing that the North is forcing its people to endure sacrifices with this provocative action and is bringing isolation and sanctions to itself from the international community," the South's Unification Ministry said in a statement.

The rocket will bisect a sea that separates South Korea and China and its flight path will take it towards the Philippines where a second stage of the rocket is due to come down in waters close to the archipelago.

China, which backs North Korea economically and diplomatically, reiterated its pleas for calm and said it had "repeatedly expressed its concern and anxiety about the developments", foreign ministry spokesman Liu Weimin told a press briefing in Beijing.

The prospect of a North Korean rocket launch has alarmed Japan, which was overflown by an earlier rocket and said it would shoot it down if it crossed its airspace.

"If North Korea launches a missile, Japan will consider the next step in cooperation with international society including the UN Security Council," Japanese prime minister Yoshihiko Noda said.

No fly zone

Some of Asia's biggest airlines say they will alter flight routes and carry more fuel because of the planned rocket launch.

Japan Airlines and All Nippon Airways say flights between Tokyo and Manila, Singapore and Jakarta are likely to be affected by the rocket launch, with flight routes shifted about 200 kilometres to the west.

Philippine Airlines will also reroute some flights and planes will carry more fuel to deal with the detours.

North Korea has warned that spent sections of its rocket will fall into the ocean west of the Korean Peninsula and east of the Philippines' island of Luzon.

Philippines authorities have declared a no-fly zone in the area, which applies to all international carriers.

You have no doubt been hearing a lot about the Paris Agreement and know that it pertains to climate change, but are too embarrassed at this stage to ask for an overall explanation of what it's all about.